i) Field of the Invention
The present invention relates to a bag for packaging heavy materials. More specifically, the invention relates to a bag for packaging heavy materials, which is made of an inorganic filler-containing multi-layer polyethylene film, and exhibits excellent strength, content packaging performance, stacking performance on the pallets and anti-collapsing performance in combination.
ii) Description of the Prior Art
Bags made of a polyethylene film having a thickness of about 100 .mu.m to 250 .mu.m have been widely used for packaging heavy materials such as fertilizers, grains such as rice and the like, chemicals, etc. in amounts of 5 kg to 30 kg. Such contents are usually packaged in the bags by using a machine. In this case, however, the contents will not be easily packaged in the bags if the bags are not favorably opened or exhibit poor rigidity and poor slipping property. Therefore, the bags must exhibit slipping property on the inner surfaces thereof.
The bags packing the contents are stacked on the pallets. To stack the bags, it is desired that the bags exhibit slipping property to some extent on the outer surfaces thereof. After the bags have been stacked, the outer surfaces of the bags must become little slippery, so that the stacked bags are not collapsed while the pallets are being moved or during the storage. With respect to slipping property on the outer surfaces of the bags, as described above, conflicting properties are required depending upon the steps.
In recent years, furthermore, packaging and stacking operations have been carried out at increased speeds for better efficiency, and the above-mentioned properties on the inner surfaces and outer surfaces of the bags have been more desired than ever before.
To meet such requirements, attempts have been made to add a slipping agent, an antistatic agent and the like agents in proper amounts to the resins constituting the film when the bags are made of a single-layer film without, however, succeeding in obtaining bags that satisfy the above-mentioned conflicting properties.
Accordingly, efforts have been made to develop bags made of a multi-layer film by changing the amounts of additives and the thicknesses of the layers. For example, Japanese Patent Publication No. 25135/1992 discloses a bag made of a three-layer film comprising an outer layer, an inner layer and an intermediate layer having a ratio that lies within a particular range, and in which a slipping agent is added to the outer layer and to the inner layer. However, the slipping agent abruptly exhibits its effects when it is added in a given amount, and it is very difficult to so adjust the amount of addition of the slipping agent that slipping property of a proper level is obtained. In the bags made of a multi-layer film, furthermore, the slipping agent migrates in the layers and, hence, the effect of the slipping agent changes with the passage of time, which makes it difficult to maintain the slipping property at a suitable level over extended periods of time.
Japanese Patent Publication No. 57223/1988 discloses a bag made of a two-layer film which is made up of two layers of resins of different kinds. However, this bag has the outermost layer exhibiting low slipping property which helps prevent the stack of bags from collapsing but arouses such problems that the contents are not packaged as desired and that the bags are not well stacked.